(1) Field of the Invention
The present invention relates to a current generating system, and more particularly to the system having a self-excitation alternating current generator used in a motor vehicle for supplying electric power to a battery, electric loads such as lamps and so on.
(2) Description of Prior Art
It is recently required for a generating system used in a motor vehicle to produce sufficient electric energy during whole operating conditions of the engine mounted in the vehicle, namely at a low speed operation as well as a high speed operation of the engine.
In this respect, it is known that electrical connections of poly-phase current generating windings of the generator are changed in response to the operating conditions thereof to obtain higher electric energy. For example, in a separate excitation generator of three-phase current generating type, the windings are arranged as three-phase Y-connected windings at the low speed operation while the windings are changed to operate as respective independent generating windings at the high speed operation, as it is disclosed in a U.S. Pat. No. 4,024,456 granted to the present inventors.
On the other hand, it is a problem of a current generating system having a self-excitation generator that a rotational speed of the generator, where it begins to generate an output of alternating current, is higher than that of the separate excitation generator. In a conventional self-excitation generator, a rotor of the generator, especially pole cores thereof are made of a material such as having a high residual magnetism, or number of winding turns of an armature is increased, in order to decrease the rotational speed of the generator where it begins to generate the output.
However, even though the material having the high residual magnetism is used for the pole cores, or special treatments are employed for the pole cores to increase the residual magnetism thereof, there exists a limit of the rotational speed decrease. Accordingly, the rotational speed of the self-excitation generator where the generator begins to generate the output is still higher than that of the separate excitation generator.
In addition when the number of winding turns is increased to increase the output energy at the low-speed operation of the generator, the output energy thereof is contrariwise decreased during the high-speed operation, with a result that the effective charging for a battery can not be obtained during the high-speed operation.